One such apparatus usually finds application as a guard in preventing, for example, part of the human body becoming trapped between the closure member and a frame enclosing the closure member at least in part. The way in which such a capacitive anti-trap guard works is based on how the electric field is influenced by an electrically conductive obstruction in the opening range of the closure member which can be sensed as a change in capacitance of the sensor electrode. Unlike a so-called tactile anti-trap guard requiring physical contact of the obstruction with a sensor, the presence of an obstruction in the opening range of the closure member with a capacitive anti-trap guard is sensed without contact.
A capacitive anti-trap guard or trap protector is described in EP 1 154 110 A2. This known anti-trap guard comprises two electrical conductors which generate the electric field in the opening range of a closure member. One of these two conductors, the sensor electrode, is integrated in a weatherseal sealing the closure member and receives a predefined electric charge. It is in this way that a reference capacitance can be defined between the sensor electrode and the other conductor, the base electrode, which, for example, is grounded. The change in capacitance of the sensor electrode relative to the reference capacitance because of the presence of an obstruction in the opening range of the closure member is detected by means of a controller which furnishes a control signal for a motor powering the closure member. To permit detecting a non-conductive material, such as wood or plastics, for example, prompting no or merely an insignificant change in capacitance, the anti-trap guard is provided with a so-called soft-spot. The soft-spot is achieved by making the portion of the weatherseal accommodating the sensor electrode deformable. In this way there is a change in the relative position of the sensor electrode and base electrode when the obstruction comes into contact physically with the weatherseal and thus a change in capacitance. The known anti-trap guard (trap protector) is characterized by an active redundance which effectively prevents the obstruction being trapped irrespective of its material.
A capacitive anti-trap guard or trap protector is furthermore described in DE 694 13 170 T2. This anti-trap guard comprises a control unit that includes an oscillator generating a signal. At a closed window pane, the signal is dependent on the surrounding conditions close to the window pane, like temperature and moisture for example. For this reason the anti-trap guard comprises a compensation switch compensating for the surrounding conditions in order to create an independent control signal.
Known furthermore are capacitive sensors which find application in determining the relative moisture content. By absorbing water the capacitance of such a capacitor made for example of a thin film of polymer metallized on both sides, is reversibly changed. Such moisture sensors thus often find application in sensing air quality, air-conditioning control or in a demister.
A capacitive moisture sensor which is put to use for detecting moisture on the windshield of a motor vehicle is described in EP 0 753 438 B1. The moisture sensor comprises a glass laminate and at least three non-grounded electrodes arranged on an inner surface of the glass laminate. The electrodes form two sensing portions on an outer surface of the glass laminate. The difference in the amount of moisture sensed in the two sensing portions is used as a reference for outputting a signal. In addition to the electrodes a shielding electrode may be provided which directs the sensitivity of the electrodes at the outer surface of the glass laminate. The drawback in this known moisture sensor has proven to be that the electrodes are directly mounted on the glass laminate, resulting in relatively complicated and cost-intensive fabrication. Apart from this, the moisture sensor can only be used on dead window panes, such as the windshield or rear window of a motor vehicle. For powered window panes as is often the case with side windows of a motor vehicle, the known moisture sensor is unsuitable since the electrodes secured to the window pane are included in the movement in thus altering the sensing portion.